1. Field of the Invention
The present invention relates to a light intensity converter capable of converting the light intensity distribution of an incident light and emitting the converted light. The present invention also relates to an optical device and an optical disk drive using such a light intensity converter.
The present invention provides an optical element in which a laser beam having a light intensity distribution in the direction perpendicular to the optical axis is converted into a laser beam having a uniform wavefront intensity in a given aperture and the transmission wavefront aberration (RMS) is reduced to 0.07 .lambda. or less, thereby making is possible to improve the beam diameter and the beam shape in a laser beam processing system, a microscope optical system and an information recording medium optical system, used in various fields.
2. Description of the Related Art
The conventional method of changing the beam intensity distribution uses a diffraction grating with the diffraction efficiency changed from one position to another, a concentration filter having a transmission distribution inversely proportional to the intensity distribution, and a slit for taking out an area of small beam intensity distribution. Various other methods have also been proposed, including a method for superposing laser beams by a half mirror or the like, a method for generating a secondary light source by a diffraction grating or the like, a method for replacing inner and outer intensity distributions with each other by refraction through a prism or the like, and a method for utilizing a blooming of a zoom lens.
Japanese Unexamined Patent Publications No. 3-75612 and No. 3-92815, for example, disclose a method for converting an incident light having a Gaussian intensity distribution into an exiting light having a substantially uniform intensity distribution, using first and second lens groups lenses with each group including two lenses.
The above-mentioned methods using the diffraction grating, the concentration filter or the slit cannot basically attain the light utilization rate of 100% and are always accompanied by an optical loss. Also, the method of superposing laser beams and the method of generating a secondary light source cause a wavefront aberration and cannot reduce the laser beam to the limit of diffraction.
In the method using refraction, on the other hand, the light paths are crossed on inner and outer sides, so the light paths are different and a wavefront aberration is developed. Also, the method using the zoom lens attains a uniform intensity at the sacrifice of an increased wavefront aberration. The conventional technologies described in JP-A-3-75612 and JP-A-3-92815 use at least four lenses while retaining the zooming function.
In the prior art, therefore, emphasis is placed on securing a predetermined intensity distribution of a beam spot rather than reducing the beam spot size. The above-mentioned methods could be used for a laser machining equipment or the like for which a spot size of about several .mu.m is allowable and the light amount loss poses no problem. The above-mentioned methods, however, cannot be used for an optical system including a low-output light source for reading data from an information recording medium because of the wavefront aberration and the light utilization rate.